In many environments, the structural integrity of certain critical components of a particular system or apparatus is of the utmost importance in ensuring against future failures. In particular, the construction of large gas turbines imposes strict requirements on performance characteristics and reliability of the parts used. For example, components used in the construction of large gas turbines are typically subjected to great mechanical stresses and must be able to withstand continued exposure to extreme high temperatures, as well as, high velocity corrosive gas streams. One example of such critical components is the rotor of large gas turbines. Conventionally, superalloys (e.g., 40 Ni--15Cr--Ti--Nb) are utilized as suitable materials for the large solid disk forgings which make up a turbine rotor. To guarantee that the superalloy material quality is high and the structural integrity of rotor forgings is acceptable, special ultrasonic tests were developed to inspect each forging in the plane perpendicular to the highest operating stresses (i.e., the axial-radial plane). In particular, two ultrasonic testing techniques are conventionally used for the nondestructive testing of turbine rotor disk forgings near the highly stressed bore region: (1) a "Pitch-Catch" technique using two ultrasonic transducers, and, (2) a "Pulse-Echo Axial LongWave" technique that uses only a single transducer.
Ultrasonic Pitch-Catch testing per se is known and is commonly used to inspect machine components for material defects oriented perpendicular to the testing surfaces. However, in the past, these tests have been performed either uncalibrated (where simply the presence, but not the size of a defect is important), or a calibration was performed by using external blocks of material containing machined reflectors of a known dimension. Calibration references of this sort for conventional Pitch-Catch testing are listed in the nondestructive testing handbook and other reference tests, such as Krautkramer's Ultrasonic Testing of Materials. Unfortunately, these attempts at calibration, or the lack of calibration fail to compensate Pitch-Catch inspection data for variations in acoustic properties that are inherently present in many superalloy materials. More specifically, many superalloys are anisotropic and present nonuniform acoustic properties as a result of an intrinsically coarse grain structure. Consequently, such variations in the acoustic properties of superalloys affect the Pitch-Catch sensitivity, as well as the ability to estimate the size of a detected defect from its reflected signal amplitude.
A further disadvantage of not calibrating a Pitch-Catch test procedure is that no size information is provided following detection of a defect. In addition, the sensitivity of the test would be unknown. Without some estimate of defect dimensions, structural analysis cannot be accurately performed to determine the severity of such a defect on the mechanical performance and reliability of the particular component part.
In the alternative, if some external material is used for calibration, the relationship established in the external reference will not be applicable to the particular forging specimen under test. This is because due to their highly nonuniform properties, no two superalloy forgings are alike. In fact, acoustic properties within the same forging may vary considerably. Therefore, an external calibration standard, even using similar material, would fail to completely characterize the specific forging specimen being tested. In addition, many combinations of forging sizes and calibration reference reflectors would be required to address all components of a particular product line. Moreover, the effective changes in engineering requirements on test sensitivities would be difficult or impossible to predict.
In accordance with the present invention, a calibration technique is provided for conventional Pitch-Catch ultrasonic testing procedures that adjusts for variations in material ultrasonic test properties. More specifically, in accordance with the calibration technique-of the present invention, "through-transmitted" sound waves are collected prior to the Pitch-Catch diagnostics on the forging and used to normalize all subsequently collected Pitch-Catch inspection data. In addition, measured amplitudes of potential fault indications ("indication amplitudes") are equated to a known reference defect (e.g., an equivalent flat bottom hole) by a particular relationship (discussed further herein) based on the through transmitted sound wave, component thickness and the ultrasonic test specimen/material characteristics.
Since the through-transmitted sound wave is affected by the material acoustic properties in the same way that the inspection test data is affected, it can be used to normalize the inspection data prior to automated defect detection or sizing. In addition, using the through-transmitted sound wave to equate the amplitude of defect indications to a known sonic reflection standard, such as an FBH reflector, allows the current test sensitivity (size of indications) to be determined uniquely for each Pitch-Catch test conducted. Using a through-transmitted sound wave in this manner provides a means to uniquely compensate for variations in acoustic properties from forging to forging as well as within any given forging.
In addition to the specific above-mentioned problems inherent to Pitch-Catch diagnostics, prior art ultrasonic inspection techniques of all types prove difficult (if not impossible) to adjust flaw/defect detection threshold levels to always obtain an optimized sensitivity for testing acoustically noisy materials. Ultrasonic inspection systems conventionally rely on an expected echo-dynamic pattern associated with the reflection of ultrasound from a defect as the sound beam traverses it. The prior art ultrasonic flaw/defect conventional flaw detection and recognition methods as employed in Pitch-Catch, Pulse-Echo or other diagnostic techniques, in general, either use a fixed predetermined amplitude as a defect recognition threshold, or use no threshold at all. In systems using fixed thresholds, the threshold is adjusted to a level that is determined by the test system operator based on a known reflector size or set to a level slightly higher than an approximation of a general (average) noise level observed in the material.
Consequently, when testing acoustically noisy components like turbine rotor disks, if the detection criterion is based solely on the expected echo-dynamic pattern from a flaw/defect, and no thresholds are used, an abnormally high number of false defect indications are encountered as a result of the nonuniform distribution of acoustic noise within the component material. This is due to the fact that acoustic noise patterns in various superalloy materials used in turbine rotors are coherent enough (i.e., have the proper echo-dynamic characteristics) to actually appear as real defect indications. Accordingly, the echo-dynamic characteristics of such materials make the use of amplitude detection thresholds absolutely necessary when testing turbine rotor forgings of other parts which use superalloy materials. However, because the noise structure in rotor forgings is typically nonuniform, not only between two separate forgings but within single forgings as well, selection of an absolute threshold value is not feasible. It would, invariably, require the setting of some arbitrary threshold quite high above observed noise peaks to eliminate false alarms. Obviously, such a setting does not provide optimum test sensitivity. Accordingly, for the purpose of testing turbine rotor forgings and other components made of superalloys or other acoustically noisy materials, it would be highly desirable to have a testing system which could dynamically optimize the test sensitivity. An improved automatic flaw/defect scanning and detection technique in accordance with the present invention solves this problem by providing the capability to dynamically adjust the flaw/defect detection threshold level so as to instantly and automatically respond to changing noise patterns. Moreover, the improved technique is readily applicable to both Pitch-Catch and Pulse-Echo diagnostics as well as other techniques, as will be recognized by one of ordinary skill in the art upon reading the following description of the invention.
In accordance with the improved automatic flaw/defect scanning and detection technique of the present invention, a "rolling average" of digital ultrasonic inspection (measurement) data is used to estimate the "local" noise level and compute a unique signal-to-noise (S/N) ratio for every data point (i.e., transducer pulse) generated. Variations in the S/N ratio for the material are measured at each axial slice of material volume inspected. A predetermined function of the standard deviation of the calculated S/N ratio values is then used to continuously adjust the detection amplitude threshold to dynamically optimize the defect detection criterion and minimize false alarms. This predetermined detection threshold function, as discussed in greater detail further herein, can also be varied to accommodate the particular type of test being performed (i. e., pitch-catch, axial long-wave, etc. ). Subsequent additional processing of sequential S/N values is then performed to determine whether a recorded internal reflection, i.e. , a "target," meets a particular predetermined criteria for a defect indication. The echo-dynamic behavior of the S/N values must, therefore, correspond to a dynamically custom-tailored "expected" response from a target to be reported as a defect indication. Thus, the automatic flaw/defect detection technique in accordance with the present invention provides increased defect detection sensitivity over conventional methods and continuous automatic adjustment of the detection threshold based on the particular intrinsic material properties of the part being inspected.